Hi Theo:
Thanks for your reply.
Yes, it is a Tiger with Jag rear suspension. The car seems (in static
position) to have about 3" of bump (upward) clearance before the hub
contacts the upper frame rail. Putting in a set of Jag bump stops was
on my to-do list when I read Mr. Puhn's book and started smelling a
problem with the whole set-up.
Currently, the 'cage' for the suspension is attached at the underside of
the framerail arch. I'm not sure I have enough room to move the cage
physically upwards much further (assuming I can deal with somehow
relocating the mounting points from the bottom of the arch upwards
towards the top of the arch) because it looks like the top of the cage
is coming into contact with the lateral piece of punched steel that runs
(I think) between where the stock Tiger has it's upper rear shock
mounts. So my vertical relocating of the cage seems to be nixed. Unless
I can move the cage forward as well as up. Which would, I think, want
to take the hubs forward with it. Which would mean shortening the drive
shaft so the 'punkin' could also come forward with it. Say, this isn't
looking real good, is it?
The bottoms of the dual coilovers mount to the lower control arms. I
had thought about relocating these mounting points downward about 2"
which would lower the car and still retain full spring and shock travel.
BUT, if I've only got 3" of bump travel before the hub contacts the
frame arch, then it seems to me I've reduced my effective bump travel to
1". It's a street car so I'd like to keep a bit more travel and retain
the relatively soft stock Jag springs. I suppose I could try to reshape
the frame arch itself (higher) to allow the hub to keep it's freedom of
movement. Messy. AAAARRRRGH! Thus my question about leaving everything
as-is (accepting that it's not a great situation) and trying to
compensate with a stiffer anti-roll bar. Which, as you pointed out,
means using a not-very-stiff chassis to compensate for this problem.
If you see something I've missed that would simplify things (other than
returning to a live axle and leaf springs), I'd be grateful.
You mentioned arched upper links. I'm sorry, but I can't quite get a
mental picture of how that works (?).
Thanks again for the reply!
Regards
David
Theo Smit wrote:
> Hi David,
> #1: Yes, if you had an independent rear suspension that consisted of Chapman
> struts or upper and lower A-arms. Semi-trailing arm rear suspension and
> swing axle rear ends have nastiness that is all their own.
> #2 and #3: If lowering the rear of the car restored the A-arm angles to what
> the designers originally intended , then you're in business. I once lowered
> a Corolla GT-S in the front by first cutting the springs, then putting
> machined spacer blocks between the lower end of the struts and the steering
> knuckle, to maintain the lower control arm angle close to stock.
> In Fred Puhn's book, lowering the car is not an option because Formula cars
> have to meet a minimum ride height requirement (either some specified
> distance or just "not hitting the pavement"), and the availability of
> shorter tires hasn't changed that restriction.
> #4: Yes, but two wrongs don't make a right. Raising the rear roll center
> causes the car to become more understeery, and stiffening up the rear
> suspension will cause it to break traction sooner, which will therefore
> induce some oversteer...under certain conditions. I'd have to think hard and
> read lots (and maybe do some testing) before deciding that the higher roll
> center and the bigger rear sway bar necessarily caused equal and opposite
> effects at all times. I would be inclined to think otherwise - better to get
> the A-arm angles where they should be, and then maybe play with spring rates
> and sway bars to do some fine tuning.
>
> If you're trying to use sway bars to shift the understeer to oversteer
> balance around, then you need to take into account that the Tiger is not a
> very rigid chassis from a torsional perspective. Going big on the bars just
> means you're using the floor pan to tune your suspension.
>
> Is yours a Tiger with a Jag rear end? What if you built some (strong) arched
> upper links so that you could set the ride height lower and still clear the
> chassis under full compression?
>
> Theo
>
> sosnaenergyconsulting wrote:
>
>
>>Hi:
>>I recently bought a copy of Fred Puhn's book "How to Make your Car
>>Handle" and had a question for y'all on the list.
>>
>>Don't have the book handy, but there's a section where he shows (using
>>an early Lotus as an example) how the independent FRONT suspension of a
>>car that was designed for large diameter tires (I think he used 26" for
>>the example) will change when smaller diameter tires and wheels (I
>>believe the example used 22")are used. The lower control arm, which is
>>designed to be more or less level when using the larger tires becomes
>>angled when using smaller tires. This causes the roll center to rise.
>>I'm nutshelling here.
>>
>>Question #1 is this: Wouldn't the same principle apply to an independent
>>REAR suspension?
>>
>>If the answer to the above question is Yes, then question #2 (ignoring
>>the front suspension which WAS designed for the smaller diameter wheels
>>and tires being currently used) is: would lowering the car at the rear
>>compensate for the higher rear roll center caused by using the smaller
>>diameter tires and wheels in the first place?
>>
>>And lastly, if the answer to the second question is Yes, my final (if
>>you believe that I have a bridge to sell you :-) ) question is: would
>>lowering the car now cause the lower rear lower control arms to become
>>more level?
>>
>>Okay, on re-reading this, I came up with question #4: If the answer to
>>question #2 is "no", or if lowering the car is not an option is it
>>possible to compensate for the higher rear roll center in some other
>>way, such as a stiffer rear anti-roll bar (possibly upgrading the front
>>bar also in order to balance things)?
>>
>>Thanks in advance for any help you folks can give me.
>>
>>Regards
>>
>>David (perpetually confused) Sosna
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